This invention relates to new bodies having infrared radiation transparency, a property not possessed by other bodies of identical chemical composition. More particularly, this invention relates to a multicomponent, multiphase, polycrystalline mass of controlled grown crystals substantially free of voids, comprising at least two different solid phases in substantially continuous atomic contact with each other. The instant grown crystals which are multicomponent and multiphase are by definition also polycrystalline.
The polycrystalline bodies of the present invention comprise multiphases and multicomponents in the sense used by Willard Gibbs as extended by Smits (see Findlay, The Phase Rule, Longman Green, New York 6th Edition 1927, pp. 6, 7, and 28). In the multiphase, multicomponent polycrystaylline bodies of the present invention, the phases differ in atomic structure or lattice spacing, and the components, of which there are more than one, conform to the definition for equilibrium (Findlay supra).
Constituents which may be used to prepare the bodies of the present invention are represented by the general formula: EQU I M.sub.m X.sub.n
Wherein M represents an element of Groups I, II, III, IV, V, and VII; X represents an anion; and m is an integer ranging from 1-6 inclusive; n is an integer ranging from 0-6 inclusive; and wherein at least two components consisting of at least one constitutent represented by said formula are capable of forming a eutectic.
The multicomponent, multiphase, polycrystalline bodies of the present invention are prepared by conventional crystal-growing techniques, used heretofore to grow single phase, essentially monocrystalline bodies from a melt. The necessary proportions of two or more constituents, capable of forming a multicomponent eutectic, are mixed intimately and grown by any one of the well known crystal growing procedures, such as the Stockbarger-Bridgman procedure (U.S. Pat. No. 2,149,076), the Kyropoulos-Czochralski procedure [Z. Phys. Chem. 92, 219 (1917)] or the Verneuil procedure (C. R. 135, pp. 791-4, 1902).
The bodies as prepared according to the instant invention are characterized by an intimate matrix of large, visible crystals in the size range from about 100 microns to about 5 mm. in cross section and arbitrary length, generally several times the cross section limited only by the length of the ingot grown. They do not cleave, are resistant to thermal shock and impact, and approach maximum density for the overall composition.
A preferred class of multicomponent, multiphase, polycrystalline bodies may be prepared according to the present invention having good to excellent transmission of light of selected wave lengths by choosing materials having very similar refractive indexes. Depending on the composition substantial percentages of specular (collimated) transmission are observed at different wave length ranges of the spectrum. In order to obtain a significant amount of specular transmission, i.e., in excess of about 20 percent transmittance, the components of the instant composition should be chosen so that the ratio of their refractive indexes falls within the range of from 1.0 to 1.2, and preferably within the range of from 1.0 to 1.05, for the larger over the smaller. For usable diffuse transmission such as is permitted in a scintillator or fluorescent light source, the range for the ratio of refractive indexes of the separate phases can be broader, including ratios as high as 1.5 or even higher.
It has been found that the fluorides of the elements of Groups II, III, IV, V, and VI of the Periodic Table (Handbook of Chemistry of Physics, 40th Edition, page 444, Chemical Rubber Publishing Co., Cleveland, Ohio) particularly are useful as constituents for preparing the light-transmitting bodies of the present invention, especially light having wave lengths in the infrared range. Almost all of these fluorides are substantially water-insoluble and, when comprising the components of a multiphase, optically integral crystal, transmit a large amount of light, particularly in the infrared range.
It is unexpected that the bodies prepared according to the present invention are capable of transmitting collimated light in view of the fact that the identical components when fused and cast as an integral body uniformly are opaque. Bodies of the present invention which are pervious to infrared radiation may then be properly termed as being optically integral. "Optically integral" is used in the sense that there is substantially an optical coupling between the parts of the whole body providing an infinite number of light paths having much the same efficiency throughout the integral body, either where light is transmitted from one surface of the body to another, as in a window, or from a point within the body to a surface of the body as in a scintillator.
Certain of these optical bodies which transmit wave energy extremely well, especially in the infrared, may be used in many areas where single crystals cannot be used because of their intrinsic nature; the crystals may bend in a certain direction, cleave under impact, fracture with thermal shock, and the like. A preferred polycrystalline body prepared according to the present invention consists of barium fluoride and calcium fluoride, each of which constitutes a separate phase from the other within the body. These materials conduct light energy extremely well in the range of wave lengths from about 1.5 microns to about 12 microns at a thickness of 5 mm.
The light-transmitting bodies of the present invention have many advantages over the pressed optics which are prepared by hot pressing small, randomly oriented, granular crystals into an integral matrix to form an otpical window. Pressed optics are used in various areas for transmitting light because they are not susceptible to cleavage. Because of this random orientation, they are more resistant to thermal shock and impact and therefore may be used under conditions which would cause crystals to break or cleave. The pressed optics in many ways are similar to bodies of the present invention by way of their physical properties. These pressed optics, however, are limited as to their degree of purity and practically as to size. Because of high surface area in the granular mass of small crystals which is used to obtain the pressed optic, it is extremely difficult to acquire a finished product of maximum purity. The mechanical confinement of the die substantially suppresses the vaporization of almost all of the light-absorbing impurities and renders any scavenger additives useless. Moreover, because the optical prisms must be hot-pressed in a die, they are limited technologically by the size of the die and press and the ability of the die to control the density throughout the entire mass of small crystals. Various glass-forming compositions have been investigated for developing infrared transmitting bodies, but the best compositions only transmit infrared having wave lengths up to 6 microns and invariably absorb infrared radiation in the 3 micron region.
The preferred barium fluoride-calcium fluoride polycrystalline bodies of the present invention advantageously may be cut or ground into various optical shapes and used with various light detecting devices and systems which are frequently subjected to shock or other rough treatment. Their size is not influenced by the expense of making a die for each particular application. Moreover, the bodies of the present invention may be made almost any size without internal strains, internal cracks, open boundaries between components and the like, whereby the bodies may be cut, ground or otherwise formed into any shape or configuration. They are particularly useful as a nose window for wave-energy guided vehicles such as the infrared homing missiles and the like. Because of their unique physical properties, the multiphase polycrystalline bodies of the present invention particularly may be adapted for such rugged use, cooperating in a unique manner (excellent optical cooperation) with infrared detecting and guiding systems employed with such vehicles and the like.